"Inevitably nature is not always exact, despite the precision of the honeycomb. When looking for 120° angles in animal forms it is important to remember another geometric law, which is that flat hexagons will only interlock in a flat plane; they cannot be combined to enclose a space, as can the triangles that constitute the tetrahedron. Where hexagons do occur on curved surfaces -- such as in the beautifully delicate skeletons of some microscopic marine organisms called Radiolaria -- there are always some other shapes and angles inserted to compensate for the curvature. The same is true of the tortoise's shell, where remarkably regular hexagons in the centre are bounded by pentagons (five-sided shapes) which fuse to give a straight edge to the shell; exactly the same happens in insect wings. Three-way junctions also tend to occur where pieces of similar size and shape must be overlapped to cover a surface, as in the feathers of a bird, the scales of a fish, or the scales of a pangolin." (Foy and Oxford Scientific Films 1982:32)Learn more about this functional adaptation.

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Wikipedia

Giant tortoise

Giant Tortoise are characteristic reptiles that are found on two groups of tropical islands: the Aldabra Atoll in Seychelles and the Galapagos Islands in Ecuador. These tortoises can weigh as much as 300 kg (660 lb) and can grow to be 1.3 m (4 ft 3 in) long. Giant tortoises originally made their way to islands from the mainland; for example, Aldabra Atoll and Mascarenes giant tortoises are related to Madagascar tortoises while Galapagos giant tortoises are related to Ecuador mainland tortoises. This phenomenon of excessive growth is known as islands gigantism or insular gigantism. It occurs when the size of the animals (especially reptiles) that are isolated on an islands increases dramatically in comparison to their mainland relatives. This is due to several factors such as relaxed predation pressure, competitive release, or as an adaptation to increased environmental fluctuations on islands.[1][2] These animals belong to an ancient group of reptiles, appearing about 250 million years ago. By the Upper Cretaceous, 70 or 80 million years ago, some had already become gigantic. About 1 million years ago tortoises reached the Galápagos Islands. Most of the gigantic species began to disappear about 100,000 years ago. Only 250 years ago there were at least 20 species and subspecies in islands of the Indian Ocean and 14 or 15 subspecies in the Galápagos Islands.[3]

Although often considered examples of island gigantism, prior to the arrival of Homo sapiens giant tortoises also occurred in non-island locales, as well as on a number of other, more accessible islands. During the Pleistocene, and mostly during the last 50,000 years, tortoises of the mainland of southern Asia (Colossochelys atlas),[4] North[4] and South America,[5] Australia (Meiolania), Indonesia,[4]Madagascar (Dipsochelys),[4] and even the island of Malta[4] became extinct. The giant tortoises formerly of Africa died out somewhat earlier, during the late Pliocene.[6] While the timing of the disappearances of various extinct giant tortoise species seems to correlate with the arrival of humans, direct evidence for human involvement in these extinctions is usually lacking; however, such evidence has been obtained in the case of Meiolania damelipi in Vanuatu.[7][8] One interesting relic is the shell of an extinct giant tortoise found in a submerged sinkhole in Florida with a wooden spear piercing it, carbon dated to 12,000 years ago.[9] Today, only one of the species of the Indian Ocean survives in the wild, the Aldabra giant tortoise (two more are claimed to exist in captive or re-released populations, but some genetic studies have cast doubt on the validity of these as separate species) and 11 subspecies in the Galápagos.

Giant tortoises are among the world's longest-living animals, with an average lifespan of 100 years or more.[3] The Madagascarradiated tortoiseTu'i Malila was 188 at death in Tonga in 1965. Harriet (initially thought to be one of the three Galápagos tortoises brought back to England from Charles Darwin's Beagle voyage but later shown to be from an island not even visited by Darwin) was reported by the Australia Zoo to be 176 years old when she died in 2006. Also, on 23 March 2006, an Aldabra giant tortoise named Adwaita died at Alipore Zoological Gardens in Kolkata. He was brought to the zoo in the 1870s from the estate of Lord Clive and is thought to have been around 255 years old when he died.[10] Around the time of its discovery, they were caught for food in such large quantities that they became virtually extinct by 1900. Giant tortoises are now under strict conservation laws and are categorised as threatened species.

The Aldabra giant tortoise lives on the remote Aldabraatoll, one of the Seychelles group of islands in the Indian Ocean. It is the only Indian ocean giant tortoise species alive today, others having become extinct soon after the arrival of human settlers (including the Seychelles giant tortoise which is now thought to be extinct in the wild, although the Aldabra giant tortoise and the Seychelles giant tortoise are so similar genetically that they are thought by some to be the same species).[10][11]

Today, the Aldabra giant tortoise is listed as an animal that is vulnerable to extinction in the wild. However, the Aldabra atoll has now been protected from human influence after having been declared a World Heritage Site, and is home to some 152,000 Aldabra giant tortoises, the world's largest population of this species. Another isolated population of the Aldabra giant tortoise resides on the island of Zanzibar, and other captive populations exist in conservation parks in Mauritius and Rodrigues. The captive breeding programmes on these other islands are trying to revive the species, and populations on them today appear to be thriving.[10]

Aldabra giant tortoises reproduce and lay up to 25 rubbery eggs between February and May. Mating is a noisy process, with the males bellowing like a bull, and chasing after the females. During copulation, the male’s tail guides his penis into position to fertilize the female, with copulation lasting for 10–15 minutes.[12] Egg are deposited into a dry, shallow nest on the ground making them particularly vulnerable to being eaten by predators. It is thought that female Aldabra giant tortoises are able to produce more than one clutch a year. The new born baby tortoises are called hatch-lings, and they emerge after an incubation period of 8 months. The baby Aldabra giant tortoises tend to all emerge during the same two week period which coincides with the arrival of the rainy season.[10] They reach sexual maturity at around the age of 30 years old.[12] Although some individuals have been known to live for more than 250 years, their live expectancy is between 80 to 120 years old.

The Aldabra giant tortoise has an dome-shaped shell which acts as protective armor for their soft, vulnerable body that lies underneath. They have an incredibly long neck which it uses to tear leaves from the branches higher up trees. The males, although not really that much bigger, are also known to weigh nearly 100 kg (220 lbs.) more than females. They are slow-moving animals with thick, short legs and round, almost flat feet that help them when they are walking on the sand.

The Aldabra giant tortoise is primarily found inhabiting grasslands and swamps on the islands of the Aldabra atoll, which forms part of the Seychelles island chain in the Indian Ocean. They once shared these islands with a number of other giant tortoise species, but many of these were hunted to extinction in the 1700s and 1800s. Although they are usually found in areas of dense low-lying vegetation they are also known to wander into more sparse and rocky regions when food is in short supply. They can also be found often resting in the shade or in a very shallow pool of water to cool themselves down from the heat.[10] Aldabra tortoises tend to spend their lives grazing but will cover surprising distances in search of food and are also very much at home on bare rock and thin soil. They can drink from very shallow pools through their nostrils, and the name Dipsochelys refers to this remarkable adaptation.[13]

The closest living relative of the Galapagos giant tortoise is the small Chaco tortoise from South America, although it is not a direct ancestor. Scientists believe the first tortoises arrived to Galapagos 2–3 million years ago by drifting 600 miles from the South American coast on vegetation rafts or on their own. They were already large animals before arriving in Galapagos. Colonizing the eastern-most islands of Española and San Cristóbal first, they then dispersed throughout the archipelago, eventually establishing about 16 separate populations on ten of the largest Galapagos Islands. Currently there are only 10 species of Galapagos Giant Tortoises left of the original 16 species. During the 16th and 17th centuries, the Galápagos were frequented by buccaneers preying on Spanish treasure ships. Filling a ship's hold with tortoises was an easy way to stock up on food, a tradition that was continued by whalers in the centuries that followed: "whaling skippers were almost lyrical in their praise of tortoise meat, terming it far more delicious than chicken, pork or beef'. They said the meat of the giant tortoise was 'succulent meat and the oil from their bodies as pure as butter, but best of all, the giants could hibernate in a ship’s damp for a year or more."[14]

These buccaneers took giant tortoises not only because of their meat and oil but because of their incredible adaption that allows this animals to survive on year without food or water. Once buccaneers, whalers and fur sealers discovered that they could have fresh meat for their long voyages by storing live giant tortoises in the holds of their ships, massive exploitation of the species began. Tortoises were also exploited for their oil, which was used to light the lamps of Quito. Two centuries of exploitation resulted in the loss of between 100,000 to 200,000 tortoises. Three species have been extinct for some time, and a fourth species lost its last member, Lonesome George, in June 2012. It is estimated that 20,000–25,000 wild tortoises live on the islands today.[15]

Galapagos tortoises are herbivorous, feeding primarily on cactus pads, grasses, and native fruit. They drink large quantities of water when available that they can store in their bladders for long periods of time. There are two main types of shell among the saddle-back and the domed shell. They both provide special adaption to different environments. The saddle-back shell tortoises are the smallest Galapagos tortoises, but present a very long neck and pairs of legs. They live on arid zone and feed on cactus. The domed shell tortoises are bigger with shorter neck and legs, they are found in the more vegetated islands and feed on grass.[16] They spend an average of 16 hours a day resting. Their activity level is driven by ambient temperature and food availability. In the cool season, they are active at midday, sleeping in in the morning and hitting the sack early in the afternoon. In the hot season, their active period is early morning and late afternoon, while midday finds them resting and trying to keep cool under the shade of a bush or half-submerged in muddy wallows.

Tortoises breed primarily during the hot season from January to May; however, tortoises can be seen mating any month of the year. During the cool season (June to November), female tortoises migrate to nesting zones ,which are generally located in low lands of the islands, to lay their eggs. A female can lay from 1-4 nests over a nesting season from June to December. She digs the hole with her hind feet, then lets the eggs drop down into the nest, and finally covers it again with her hind feet. The number of eggs ranges from 2-7 for saddle-backed tortoises to sometimes more than 20-25 eggs for domed tortoises. The eggs incubate from 110 to 175 days (incubation periods depend on the month the nest was laid, with eggs laid early in the cool season requiring longer incubation periods than eggs laid at the end of the cool season when the majority of their incubation will occur at the start of the hot season). After hatching, the young hatchlings remain in the nest for a few weeks before emerging out a small hole adjacent to the nest cap. Usually the temperature of the nest influences on the sex of the hatchling. Warm temperatures would yield more females while colder temperatures would yield more males.

The Mascarene Islands of Mauritius, Réunion and Rodrigues once harboured five species of giant tortoise, comprising two species occurring on Mauritius, another two on Rodrigues, and one on Réunion. The tortoises were unique to these islands and had gained a number of special adaptations in the absence of ground predators. They differed from any other giant tortoise species because of their modified jaws, reduced scales on the legs and shells averaging just 1mm thick. The shells of the giant tortoises were open-ended; the name Cylindraspis actually means cylinder-shaped. This was a specific adaptation in response to the lack of predators, where thick, heavily armored shells were no longer necessary.

Around the 16th century with the human arrival and the subsequent introduction of domestic species particularly pigs, the tortoises were rapidly hunted to extinction. Unfortunately, the thin shells were of no protection against these new invaders, pigs, rats, and cats devoured the eggs and young, and thousands were collected alive for provisioning ships. Sometimes they were even hunted for their oil, which was very vaulable around that time because it provided a cure for many ailments including scurvy.

On Mauritius, the giant tortoise disappeared from the mainland by the end of the 17th century and the very last tortoises survived until the 1730s on the islets in the north. Around the late 1800s, large number of tortoises bones were discovered in the Mare aux Songes excavations. These resulted in the description of the two species of giant tortoise endemic to Mauritius, the saddle-backed Mauritius (Cylindraspis inepta) and the domed Mauritius (Cylindraspis triserrata).[19]

Today all we have from these five species are a number of fossil bones and shells, a few drawings of live animals, and one stuffed saddlebacked Rodrigues giant tortoise in France's National Museum of Natural History.[20]

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Tortoise

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Tortoises (Testudinidae) or land turtles are a family of land-dwelling reptiles of the order of Turtles (Testudines). Like their marine cousins, the sea turtles, tortoises are shielded from predators by a shell. The top part of the shell is the carapace, the underside is the plastron, and the two are connected by the bridge. The tortoise has both an endoskeleton and an exoskeleton. Tortoises can vary in size from a few centimeters to two meters. Tortoises are usually diurnal animals with tendencies to be crepuscular depending on the ambient temperatures. They are generally reclusive animals.

Use of the term "tortoise"

Adult male tortoise, South Africa

Although the word "tortoise" is used by biologists in reference to the family Testudinidae only, in common usage it is used to describe many land-dwelling turtles. The inclusiveness of the term depends on the variety of English being used.

British English normally describes these reptiles as "tortoises" if they live on land.

American English tends to use the word "tortoise" for land-dwelling species, including members of Testudinidae, as well as other species such as box tortoises, though use of "turtle" by default is as common.

Australian English uses "tortoise" for terrestrial species, including semi-aquatic species that live near ponds and streams. Traditionally a "tortoise" has feet (including webbed feet) while a "turtle" has flippers.

Biology

Birth

Young tortoise

Female tortoises dig nesting burrows in which they lay from one to thirty eggs.[2] Egg laying typically occurs at night, after which the mother tortoise covers her clutch with sand, soil, and organic material. The eggs are left unattended, and depending on the species, take from 60 to 120 days to incubate.[3] The size of the egg depends on the size of the mother and can be estimated by examining the width of the cloacal opening between the carapace and plastron. The plastron of a female tortoise often has a noticeable V-shaped notch below the tail which facilitates passing the eggs. Upon completion of the incubation period, a fully-formed hatchling uses an egg tooth to break out of its shell. It digs to the surface of the nest and begins a life of survival on its own. Hatchlings are born with an embryonic egg sac which serves as a source of nutrition for the first 3 to 7 days until they have the strength and mobility to find food. Juvenile tortoises often require a different balance of nutrients than adults, and therefore may eat foods which a more mature tortoise would not. For example, it is common that the young of a strictly herbivorous species will consume worms or insect larvae for additional protein.

Lifespan

There are many old wives tales about the age of turtles and tortoises, one of which being that the age of a tortoise can be deduced by counting the number of concentric rings on its carapace, much like the cross-section of a tree. This is not true, since the growth of a tortoise depends highly on the accessibility of food and water. A tortoise that has access to plenty of forage (or is regularly fed by its owner) will grow faster than a Desert Tortoise that goes days without eating.

Tortoises generally have lifespans comparable with those of human beings, and some individuals are known to have lived longer than 150 years. Because of this, they symbolize longevity in some cultures, such as China. The oldest tortoise ever recorded, and one of the oldest individual animals ever recorded, was Tu'i Malila, which was presented to the Tongan royal family by the British explorer Captain Cook shortly after its birth in 1777. Tui Malila remained in the care of the Tongan royal family until its death by natural causes on May 19, 1965. This means that upon its death, Tui Malila was 188 years old.[4] The record for the longest-lived vertebrate is exceeded only by one other, a koi named Hanako whose death on July 17, 1977 ended a 226 year life span.[5]

The Alipore Zoo in India was the home to Adwaita, which zoo officials claimed was the oldest living animal until its death on March 23, 2006. Adwaita (sometimes spelled with two d's) was an Aldabra Giant Tortoise brought to India by Lord Wellesley who handed it over to the Alipur Zoological Gardens in 1875 when the zoo was set up. Zoo officials state they have documentation showing that Adwaita was at least 130 years old, but claim that he was over 250 years old (although this has not been scientifically verified). Adwaita was said to be the pet of Robert Clive.[6]

Harriet, was a resident at the Australia Zoo in Queensland from 1987 to her death in 2006, it was believed that she was brought to England by Charles Darwin aboard the Beagle and then on to Australia by John Clements Wickham[7][8]. Harriet died on June 23, 2006, just shy of her 176th birthday.

Sexual dimorphism

Many species of tortoises are sexually dimorphic, though the differences between males and females vary from species to species. In some species, males have a longer, more protruding neck plate than their female counterparts, while in others the claws are longer on the females. In most tortoise species, the female tends to be larger than the male. Some believe that males grow quicker, while the female grows slower but larger. The male also has a plastron that is curved inwards to aid reproduction. The easiest way to determine the sex of a tortoise is to look at the tail. The females, as a general rule have a smaller tail which is dropped down whereas the males have a much longer tail which is usually pulled up and to the side of the rear shell.

General information

Giant tortoises move very slowly on dry land, at only 0.17 miles per hour (0.27 km/h).[11]

Diet

Most land based tortoises are herbivores, feeding on grazing grasses, weeds, leafy greens, flowers, and some fruits. Pet tortoises typically require a diet based on wild grasses, weeds and certain flowers. Certain species occasionally consume worms or insects, but too much protein is detrimental as it can cause shell deformation and other medical problems. Cat or dog foods should not be fed to tortoises, as these do not contain the proper balance of nutrients for a reptile; in particular, they are too high in protein. Additionally, it should not be assumed that all captive tortoises can be fed on the same diet. As different tortoise species vary greatly in their nutritional requirements it is essential to thoroughly research the dietary needs of your individual tortoise. Commercial tortoise pellets should not be offered as despite their claims they are not formulated to meet the nutritional needs of any type of tortoise. Large pellets have been known to rupture the oesophagus which begins the onset of choking and can cause a slow painful death. The best approach to determining the proper diet is to consult a qualified veterinarian specialising in chelonian care.

Taxonomy

The following species list largely follows Rhodin et al., 2010[12], this is a work in progress.

In religion

In Hinduism, Kurma (Sanskrit: कुर्म) was the second avatar of Vishnu. Like the MatsyaAvatara also belongs to the Satya Yuga. Vishnu took the form of a half-man half-tortoise, the lower half being a tortoise. He is normally shown as having four arms. He sat on the bottom of the ocean after the Great Flood. A mountain was placed on his back by the other gods so that they could churn the sea and find the ancient treasures of the Vedic peoples. Tortoise shells were used by ancient Chinese as Oracle Bones to make predictions.